A pick-up arm for a phonograph generally carries a cartridge having a stylus one end of which is connected to the main body of the cartridge by means of compliant coupling, the other end resting between the two walls of a record groove during use. Information in the groove is converted into electrical signals in dependence upon the relative vertical and horizontal velocities between the stylus tip and the main body of the cartridge. The arm should therefore carry the cartridge in such a way that the cartridge converts into electrical signals all of the recorded audio information, and no other extraneous information such as record warps, eccentricity and turntable rumble. In order to achieve these conditions, the arm should be completely motionless (that is apart from the movement necessary to track the spiral record groove) when the cartridge is detecting audio information (i.e. frequencies in the range from 20 Hz to 20 kHz, this range being generally known as the audio bandwidth). If the arm moves when it should not there is a loss of definition causing such effects as, for example, brash sounding guitar, muddling of base, or screeching of violin tones. Such unwanted movements in the audio bandwidth can be caused by reflections of some of the mechanical energy which is not converted into electrical signals.
However, it is necessary for the pick up arm to possess such freedom of movement that the stylus is able to follow gross errors of movement of the point of contact between the stylus and the groove on the surface of the record--such as those due to warps and eccentricity in the record, and mechanical inaccuracies in the turntable used to support the record. The error can be of up to 5 mm magnitude; the modulation of the groove itself typically ranges from 0.1 .mu.m to 0.5 mm.
Eccentricity errors in records cause unwanted movements within the horizontal plane having frequencies of 0.56 Hz and 0.75 Hz respectively for standard speeds of rotation of micro-groove records, i.e. 331/3 and 45 rpm.
The frequency of the unwanted movements within the vertical plane, which are due principally to warped records, depends on the number of warps. It can be calculated that the fundamental frequency of such movements ranges between 0.56 and 3 Hz, the maximum occurring in the unusual case of a record with four warps, played at 45 rpm. The maximum fundamental error frequency in either horizontal or vertical planes is therefore well below 20 Hz, that is well below the lowest frequency of the audio bandwidth, although harmonics can exceed 20 Hz in frequency. There will be a frequency dependent on the combination of the stiffness of the stylus mounting and the mass of the arm and cartridge and in the absence of damping at which a fundamental resonance condition can occur if an input from a record, either from warps or groove modulation, occurs at, or close, to this frequency. In the absence of damping the assembly can vibrate with an amplitude many times greater than the amplitude of the original input and large distorting signals are produced by the cartridge. Such distorting signals can vary from about 5 Hz to 20 Hz. It is of course desirable that such resonances do not occur in the audio bandwidth but even those at subaudible frequencies tend to modify the higher frequency audible signals, by cross-modulation, and can produce a cartridge amplifier overload condition which can damage both the amplifier and other equipment such as loud speakers, and also lead to excessive record wear. Hence a mechanical filter or damping means is needed to ensure that the pick up arm will allow gross movements of the pick up, and hence of the cartridge body, at frequencies below the audio bandwidth, but will minimise all unwanted movement of the cartridge at frequencies within the audio bandwidth and will damp any resonance of the arm and cartridge.
Damping has been used in SME equipment, in which there is a paddle, rigidly attached to the arm near its pivot, sitting in a stationary bath of high viscosity fluid fixed to the support on which the pivot rests. The bath is arcuate having a radius sufficient to provide equal damping at all playing positions. The Hadcock GH 228 super arm, Keith Marks M9BA mark 3 and Mayware formula 4 arms also use viscous damping in a manner very similar to this. In these known cases the damping means is applied either at the bearing itself or in close proximity thereto and although they allow free movement of the cartridge over warps, at higher frequencies but within the audio bandwidth, more complex vibration modes of other arm components (sometimes caused by acoustical feedback) e.g. flexural or torsional vibrations of the arm tube, become evident. The damper, as well as the pivot, acts as a rigid fixed point at high frequencies, and the dynamic model of the arm changes from a pivoted damped beam at low frequencies, to an undamped cantilever beam at high frequencies. Hence the SME damper and similar models are only truly effective in low frequency applications.